Differential Mount Repair

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Andrew in Calgary
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Joined: Wed Jul 26, 2006 8:13 pm
Location: Calgary, Alberta

Differential Mount Repair

Post by Andrew in Calgary » Mon Nov 13, 2006 11:36 pm

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<title>Differential Mount Repair </title>
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<h4 align="center"><a href="http://www.firstfives.org">FirstFives.org</a></h4>

<h1 align="center">Differential Mount Repair</h1>

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<p>
Courtesy of <a href="mailto:audserv@exis.net">Peter Florance</a> </p>

<p><img src="/faq/diffmount/bigproblem.jpg"></p>

Here's what can happen if the problem is not corrected before the
frame rail fails completely. If the car feels "springy" when you
accelerate, put it on stands and inspect the differential mount
carefully.

<p><img src="/faq/diffmount/problem.jpg"> </p>

<p>Here's the basic problem. The rear of the differential is
supported by a rubber bushing. The bushing is attached to the
sheetmetal unibody box via 4 10mm bolts screwed into nuts welded
to inside of the unibody box section. The same torque that causes
e12's to 'squat' upon acceleration, applies a pulling force to
the back of the differential. Eventually with enough force the
sheetmetal fatigues and the nuts start to pull away from the rest
of the sheet metal. Mine first failed when the car was about 7
years old. You can see evidence of the mig welding that was done
by a local body shop. As the shop did not remove the bushing (I
didn't know that at the time) they were unable to weld the cracks
completely. There was a repair kit available for this problem but
I was unaware of it at the time of the first repair. It is now
NLA from BMW but JustBMWS has a few in stock. Karl gave me the
idea of supporting the sheetmetal via bolts coming down from the
trunk floor and using nuts on the diff mount bushing itself. </p>

<p><img src="/faq/diffmount/floorrecess.jpg"></p>

<p>Although I had pictures of the bottom part of the repair, I
didn't know what Karl's repair looked like on the top. I did know
that the rear mounting nuts (the ones that fail) were directly
below the rear edge of the sunken in area of the trunk. The edge
is at about a 45 degree angle so there's no easy way to just use
bolts alone. Karl did use some sort of plate to provide a flat
surface. </p>

<p><img src="/faq/diffmount/testfit.jpg"></p>

<p>I designed a plate that would fit in the sunken-in area. It
has an 'L' shaped cross section. I'll provide dimensions later (yeah,
sure). The plate is shaped and beveled to fit flush in the area.</p>

<p><img src="/faq/diffmount/plate2.jpg"></p>

<p>Here's a closeup of the plate. Note the beveling. It's a good
idea to make the plate out of something no thicker than 1/16"
if you want to MIG weld it into place as I did. Welding very
thick metal to very thin metal is very difficult to do well. I
used 1/16". I feel like I should have beveled the top as
well to make the welding easier. </p>

<p><img src="/faq/diffmount/fix.jpg"></p>

<p>Here's the plate welded into place with the bolts (M10-1.5 x
190mm long). Note the brazing on the right side. That's where I
melted a hole in the trunk. Doh! What a difference! I'm sure the
mount has been hosed for quite a while. The car feels a lot more
solid now. The bolts are threaded from the top into the mount
holes and then snugged up. Don't tighten too much or you will
collapse the box section. Note with the box deformed like mine it
was really tough to get the bolts to catch the threads. Since I
did this work I found one of the repair kits for the bottom. You
could use one of those alone but I'm happier with the idea of
both. Once that comes in and is installed I will seal around the
bolt holes with seam sealer and rustproof up in the box section.</p>


<p>3/28/2001 Here are some pictures of Karl Koenig's repair using
the factory repair kit and through-bolts (Karl was my source for
this method).<br>
<img src="/faq/diffmount/KDmount1.jpg">
<img src="/faq/diffmount/KDmount2.jpg">
</p>

<p>06/10/2004 Here are some before and after pictures of the
differential mount in Bob Szlasie's '80 633i. Dinan Engineering
did the repair work.
<img src="/faq/diffmount/BobSzlasie633iDiffMount_02.jpg">
<img src="/faq/diffmount/BobSzlasie633iDiffMount_01.jpg"></p>
</p>

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